The tunable magnetic and microwave absorption properties of the Nb5+–Ni2+ co-doped M-type barium ferrite
M-type barium ferrites with Nb5+–Ni2+ ions substituting for Fe3+ ions (BaFe12−2xNbxNixO19, BFNNO, x = 0–0.8) were prepared using the sol–gel method. The results show that barium ferrite with hexagonal plate-like grains is obtained in the samples with x = 0–0.8 sintered at 1350 °C for 3 h. The concentration of Fe2+ ions increases by ∼24% from 15.93% with x = 0 to 19.70% with x = 0.8. The anisotropic field Ha and coercive field Hc are adjusted freely from 15.75 kOe and 1.00 kOe to 6.04 kOe and 80 Oe with increasing x from 0 to 0.7–0.8, respectively. Meanwhile, the saturation magnetization, Ms, still maintains high values of >50 emu g−1. Multiple magnetic resonance peaks and two reflection loss (RL) peaks appear in BFNNO, which shift from >40 GHz to <18 GHz with increasing Nb5+–Ni2+ ions. The broad bandwidth (BW) of 4.5 to 9.3 GHz, low RLmin of −17.4 to −31.8 dB and tunable effective absorption frequency range (EAFR, RL < −10 dB) from >40 GHz to <18 GHz are achieved in BFNNO under a matching thickness (dmp) of 0.85–1.45 mm with x = 0–0.8. This indicates that BFNNO is a competitive absorbing candidate which can operate over wide frequency ranges.